The present invention relates to over-the-air (OTA) testing of radio frequency transceiver systems, and in particular to testing to detect faulty elements in an active antenna array of an extremely high frequency (EHF) wireless communication device.
As mobile wireless communication devices have become more widely used for many purposes, availability of sufficient signal bandwidth to accommodate the many varied uses (e.g., streaming of video and/or more uses of video in two-way communications in particular), has become a critical issue. This has led to more use of higher signal frequencies, such as extremely high frequency (EHF), which is the International Telecommunication Union (ITU) designation for radio frequencies in the electromagnetic spectrum band of 30-300 gigahertz (GHz), in which radio waves have wavelengths of 10-1 millimeter, and are often referred to as millimeter wave (mmW) signals.
For various reasons, including short line-of-sight signal paths due to high atmospheric attenuation, such devices often use active array antennas to beamform the signals to maximize signal path lengths (as well as to better enable frequency reuse). As is known in the art, such antenna structures include multiple active antenna elements, typically arranged in a regular array, e.g., a rectangular array of 16 or 25 antenna elements (for radiation and reception of respective electromagnetic signals) disposed in a 4×4 or 5×5 array, respectively. Accordingly, when testing such devices, it is important to be able to test each of the active antenna elements (e.g., all 16 or 25 antenna elements for a 4×4 or 5×5 array, respectively) to ensure compliance of the device to its design and/or performance specifications.
Current conventional testing techniques include performing far-field, compact range and near-field measurements of radiated energy from the active antenna elements. The far-field method is often used for testing performance of antennas that are generally used for communication between two devices that are far apart, e.g., several λ apart (where λ is the wavelength of the carrier frequency of the radiated signal). With this method, the receiver, or range. antenna and the antenna under test (AUT) are separated by a range distance R of at least R=2D2/λ apart from each other (where D is largest aperture dimension of the two antennas). For an antenna with a large aperture (e.g., several wavelengths in size), the range distance R can be large and dimensions of the shielded test chamber using such a method will be large. Hence, A test system using a far-field test method is undesirable for use in a manufacturing environment due to its size.
Further, while a far-field method may enable measuring of overall antenna performance and capturing the antenna radiation pattern, it cannot reliably detect defective elements in an antenna array since no reasonably detectable radiation difference would be observed when measuring a fully active array with a minority of defective elements (e.g., a 5×5 element array with three of the 25 elements being defective). For example, using a single-point measurement of radiated energy steered in a broadside direction from such an antenna array does not reveal a significant difference (<1 dB) from that measured from an antenna array having no defective elements. Moreover, even if such a small difference can be reasonably detected and measured, neither the number nor identities of the defective elements will be known, and even with no significant difference in measured performances of an antenna array with defective elements when steering at broadside, performance degradation may show up at other steering angles.
The compact range method, though similar in some respects to the far-field method, differs in that an apparatus is used to transform a spherical wave into planar wave within a near-field region of the AUT, e.g., by using a reflector with a complex shape designed for such purpose. However, while the compact range method helps decrease the size of the required testing envelope (as well as the shielded test chamber), like the direct far-field method, this method still cannot detect and identify faulty elements in an array in full active mode of operation.
Meanwhile, conventional near-field methods include near-field measurements that capture complex signals using planar, cylindrical or spherical scans, and simple coupling techniques that capture power magnitude only. Near-field capturing of complex signals, generally in the radiating near-field region, advantageously includes complex data that can be mathematically transformed out to the far-field region to obtain far-field performance characteristics or transformed back to the antenna surface to help perform antenna diagnostics. While such systems also have smaller footprints than direct far-field and compact range systems, they generally use a single probe to perform a measurement scan using a robot arm and, therefore, involve long test times to obtain measured data within the tested scanning surface (e.g., planar, cylindrical or spherical). While an electronic switched electronic array may be used in place of a mechanical device to accelerate the measurement scan, when a large scan is needed the necessary large switch array and design can be complex and expensive.
Simple near-field coupling techniques that capture power magnitude only, which tend to be simple and low cost and often used in manufacturing environments, use a coupler, or antenna, placed near the AUT to capture the radiated power. A comparison power test with a measured power from a reference, or known good, AUT is used to validate whether the AUT is defective or not. In order to capture all potential defects, the aperture of the coupler needs to be as large as the AUT. For a small (e.g., 2×2) array, detecting which element is defective is not critical so long as it can be determined whether the array as a whole is defective or not. However, for an AUT with a large number of elements, design of a large coupler, essentially an antenna with a very large aperture, though complex, is needed since near fields of all the elements must be measured to ensure accurate detection(s) of defective array elements. Further, such coupling method cannot identify individual defective elements in a large array when under normal operation (fully active array). While such coupling method may nonetheless be used to test on an individual element-by-element basis to detect individual faulty elements, this becomes increasingly time intensive and still does not enable testing of the array under normal (fully active) operation.